1. Field of the Invention
This invention relates to a lens barrel in which a lens is driven by a vibratory-wave motor.
2. Description of the Related Art
Various proposals for using a vibratory-wave motor as a drive source for driving the lens have already been made (in, for example, U.S. Pat. No. 4,793,685 and U.S. Patent Application Serial No. 261,265 filed on Oct. 24, 1988; 103,303 filed on Oct. 1, 1987; U.S. Pat. No. 4,660,933, for example).
In a lens barrel of this kind, for driving the lens, if the vibratory-wave drive device using an electrostrictive element is utilized, a simplification of the structure of construction, a minimization of the size, etc. can be achieved on one hand and such a problem as follows arises on the other hand. That is, the vibratory-wave drive device has a stator in the form of a vibrator and a rotor which are pressed against each other to exert a frictional force which is made use of as the drive transmission source. In the manual mode (focusing manually), therefore, a large force is required for moving the rotor against the frictional force. In other words, the conventional lens drive device when switched to the manual mode necessitates a torque which overcomes the aforesaid frictional force. For this reason, there has been a requirement for reducing it to improve the manageability.
Therefore, many improved techniques have been proposed. For example, in U.S. Pat. No. 4,864,344 the rotation of the manual ring is electrically detected, and the amount of rotation of this ring is translated to the time for which the vibratory-wave drive device is driven.
Another method of reducing the frictional torque is to cause the vibrator in the form of a vibrating ring of the stator to vibrate in a stationary wave, which allows for reduction of the contact area of the rotary ring for the rotor with the vibration ring for the stator. However, in both methods, even in the manual mode, the electrostrictive element has to be energized. This leads to an increase in the consumption of electric power. Therefore, the camera must accommodate an electric power source of increased capacity.
Meanwhile, it has been proposed in U.S. Pat. No. 4,660,933 to make use of a simple clutch mechanism arranged so that when switching the camera to the manual mode, the vibration ring for the stator and the rotation ring for the rotor are taken out of frictional contact with each other. The use of this technique, however, makes it difficult to re-establish the constant frictional force between the stator and the rotor when the camera is switched to the automatic mode. In the vibratory wave drive device, the instability of the frictional force between the stator and the rotor gives cause for lowering the drive efficiency and for generating noise during operation.